1. Field of the Invention
The present invention relates to a solid-state imaging apparatus and a driving method thereof.
2. Description of the Related Art
In recent years, a solid-state imaging apparatus has been widely used for a moving image, a still image, and other various imaging uses. In particular, the improvement of the performances of a complementary metal oxide semiconductor (CMOS) sensor, which includes an amplifying metal oxide semiconductor (MOS) transistor in each pixel and can mount peripheral circuitry on a chip is remarkable, and the improvement of the image quality of the CMOS sensor knows no boundaries. In order to further improve the image quality of the CMOS sensor, it is an important problem to reduce the random noises and fixed noises that arise in the amplifying MOS transistors in the pixels.
The technique of reducing reset noises of each amplifying MOS transistor by means of a correlated double sampling technique has been widely used. The correlated double sampling is described below. The technique obtains the difference between an output of each amplifying MOS transistor in the state in which the input portion thereof is reset (hereinafter referred to as a reset output) and an output of the amplifying MOS transistor in the state in which signal charges of a photoelectric conversion element are transferred to the input portion of the amplifying MOS transistor (hereinafter referred to a signal output). This technique of reading the signal charge quantity of a photoelectric conversion element in low noise is the correlated double sampling. By the use of this technique, the image quality of the CMOS sensor has been remarkably improved.
Moreover, the progress of the technique of the solid-state imaging apparatus aims the miniaturization of optical sizes and the increment of the number of pixels, and the pixel pitches keep on decreasing. Consequently, it is also an important object to maintain the signal charge quantity of a photoelectric conversion element even if the pixel pitches are decreased.
As a measure of achieving this object, the technique of securing the area of a photoelectric conversion element by a pixel sharing technique has been used. The pixel sharing technique is the technique of sharing the inter pixel circuits, such as an amplifying MOS transistor and a reset MOS transistor, with a plurality of photoelectric conversion element to reduce the number of elements per photoelectric conversion element. A configuration of a pixel circuit using the pixel sharing technique and a driving method thereof are disclosed in Japanese Patent Application Laid-Open No. 2005-198001.
On the other hand, 1/f noises are also known as an important factor of the noises arising in an amplifying MOS transistor. Various researches have been made with regard to the 1/f noises of a MOS transistor, and Japanese Patent Application Laid-Open No. 2003-032554 describes especially with regard to the behavior of the 1/f noises in a solid-state imaging apparatus.
The phenomenon described in Japanese Patent Application Laid-Open No. 2003-032554 can be considered as follows. According to the study of the inventor of the present invention, the influences of a variation of a bias in the output of a transistor remain for a relatively long time, of the degree of 100 ms, when the variation has arisen in the bias of the transistor. Furthermore, as a result of the research of the present inventor based on this concept, it was found that the output value of an amplifying MOS transistor minutely varied with a very long time constant in comparison with the time constant of the equivalent circuit of the amplifying MOS transistor after the gate potential thereof had greatly varied.
If the correlated double sampling is performed in a time period during which the output value of an amplifying MOS transistor is minutely varying, then the minute variation component is superimposed on a normal signal as a pseudo signal. If the amplitude of the pseudo signal disperses every pixel, the dispersion becomes a fixed pattern, and if the amplitude has a tendency in an imaging plane, the tendency becomes shading. These variations become factors of degradation of an image quality.